1. Field of the Invention
The present invention relates to a sheet folding apparatus for folding a set of sheets into two at the center.
2. Description of the Related Art
Conventionally, some of image forming apparatuses such as a copying machine or printer have a function called a booklet process. In the booklet process, the center of the sheets in which the image is formed is stapled and the set of sheets is folded into two at the center.
In the image forming apparatus having this kind of function, while the set of sheets stapled at the center is delivered to the nip portion where the pair of rotating rollers is pressed against each other, the set of sheets is folded into two at the center by passing the set of sheets through the nip portion (for example, see Japanese Patent Laid-Open Publication No. 11-332734).
However, in the image forming apparatus proposed by the above reference, forming an image on the sheet sometimes decreases frictional force between a sheet in direct contact with the rotating roller and a sheet beneath the sheet in direct contact with the rotating roller. In this case, it is reported that only the sheet in direct contact with the rotating roller is stripped from the set of sheets and thereby taken into the nip portion ahead of other sheets.
As a countermeasure, it is proposed to use a blade member to deliver the set of sheets to the nip portion by pushing them in. In this proposal, a sheet in direct contact with the rotating roller and a sheet beneath the sheet in direct contact with the rotating roller are caused to come into contact with each other by pressure of the blade member, which prevents the sheet in direct contact with the rotating roller from being stripped from the set of sheets and taken into the nip portion ahead of other sheets.
However, only by pushing the set of sheets in with the blade member, a trace of the blade member may remain on the sheet depending on the number of sheets included in the set of sheets.
The above problem is generated not only in the case where the blade is pressed against paper, but also in the case where the blade is pressed against sheet-like medium other than paper. Further, the problem is also generated in the set of sheets which is not stapled.